The multidrug resistance protein (MRP) is a membrane protein that mediates altered transport of cytotoxic drugs. Although MRP overexpression has been described in doxorubicin-selected human tumor cell lines, the murine PC-V10 and PC-V40 cell lines are members of the only reported series of vincristine-selected cell lines that overexpress mrp. Western blotting, using an antiserum developed against human MRP, demonstrated high-level expression of murine MRP primarily in the plasma membranes in each of the vincristine-selected cell lines. Only PC-V160, selected for high level resistance, demonstrated concomitant overexpression of the P-glycoprotein. As compared with parental cells, each of the drug-selected cell lines demonstrated an energy-dependent, decreased net accumulation of vincristine without any changes in the initial rates of vincristine influx. However, there was an enhanced rate of vincristine loss, 2.3-fold from the PC-V40 cell line and 3.9-fold from the PC-V160 cell line. Selective plasma membrane permeabilization with digitonin equalized vincristine accumulation among the parental, the PC-V40, and the PC-V160 cell lines. No intracellular pH differences were detected among the cell lines. Despite high-level MRP expression, daunorubicin accumulation and the rate of daunorubicin loss in the PC-V40 cells were the same as that observed in parental PC4 cells. Fluorescence microscopy demonstrated no difference in the pattern of subcellular daunorubicin accumulation between parental and PC-V40 cells. These studies demonstrate that murine MRP, overexpressed and found predominantly in the plasma membrane of vincristine-selected PC-V40 cells, is associated with an energy-dependent increased efflux of vincristine, but not with efflux or altered distribution of daunorubicin.